About Optics & Photonics TopicsOSA Publishing developed the Optics and Photonics Topics to help organize its diverse content more accurately by topic area. This topic browser contains over 2400 terms and is organized in a three-level hierarchy. Read more.

Topics can be refined further in the search results. The Topic facet will reveal the high-level topics associated with the articles returned in the search results.

Abstract

A time gating system has been constructed that is capable of recording high quality Raman spectra of highly fluorescing biological samples while operating below the photodamage threshold. Using a collinear gating geometry and careful attention to power conservation, we have achieved all-optical switching with a one picosecond gating time and 5% peak gating efficiency. The energy per pulse in this instrument is more than 3 orders of magnitude weaker than previous reports. Using this system we have performed proof-of-concept experiments on a sample composed of perylene dissolved in toluene, and the stem of a Jasminum multiflorum plant, the latter case being particularly important for the study of plants used in production of cellulosic biofuels. In both cases, a high SNR spectrum of the high-wavenumber region of the spectrum was recorded in the presence of an overwhelming fluorescence background.

Figures (5)

(a) Schematic diagram of the Kerr gating system. The pump beam path is shown as a solid red line, while the SHG path is shown as a solid navy line. The path where Raman and Fluorescence are overlapped is shown in navy with a dash-dotted line style, while the path where the Fluorescence has been temporally filtered out is shown in navy with a dashed line style. Abbreviations as follows: BPF, band pass filter; CCD, charge-coupled device; DCM, dichroic mirror; FI, Faraday isolator; λ/2, half wave plate; LPF, long-pass filter; NLM, nonlinear medium; P, polarizer; SHG, second harmonic generation crystal. (b) Diagram of pulse propagation through the Kerr shutter. The pump beam is shown in red, while the raman and fluorescence signals are shown in green and blue, respectively. A diagram of the orientations of the polarizations of the three beams at several locations is shown above the beam path. Note that the polarizations are linear at all locations and the elliptical shape is for figure clarity only.

Top: Raw spectra of perylene dissolved in toluene. Red curve shows the spectrum taken with the gate held open (analyzer set for maximum transmission). Black curve shows the spectrum taken with the analyzer aligned for minimum transmission and a pump beam applied (the gated spectrum). Green curve shows the spectrum taken with only the pump beam applied. Blue curve shows the spectum taken with the analyzer aligned for minimum transmission and no pump beam applied (gate held closed). Dashed magenta lines indicate spectral region shown in panel below. Bottom: Spectra of perylene dissolved in toluene after fluorescence background subtraction. Red curve is the spectrum with the gate held open, and the blue curve is the gated spectrum. Black curve is a spectrum of pure toluene as a reference. The gated spectrum clearly shows the high wavenumber peaks of toluene.

Top: Raw spectra of a star jasmine stem. Red curve shows the spectrum taken with the gate held open (analyzer set for maximum transmission). Black curve shows the spectrum taken with the analyzer aligned for minimum transmission and a pump beam applied. Green curve shows the spectrum taken with only the pump beam applied. Blue curve shows the spectum taken with the analyzer aligned for minimum transmission and no pump beam applied. Dashed magenta lines indicate spectral region shown in panel below. Bottom: Spectra of a star jasmine stem after fluorescence background subtraction. Red curve is the spectrum with the gate held open, and the blue curve is the gated spectrum. The gated spectrum clearly shows the characteristic high wavenumber peak of cellulose.

A curve showing the theoretical evolution of shot-noise-limited signal-to-noise versus integration time for the Raman and fluorescence strengths observed in the star jasmine stem experiment. The gated spectrum, acquired in 20 minutes, had a SNR of 118. To achieve a comparable SNR without optical gating (and assuming shot noise is the dominant noise source), one would have to acquire for 540 minutes.